Processes and apparatus for application of polymer coatings onto moving webs have long been used in the manufacture of: carpets, fabrics (such as waterproof fabrics), decals, flexible composites, and many other commercially important articles. Coating polymeric layers onto web-type substrates has long been an area of great technological interest. Common coating methods include: dip coating, rod coating, knife coating, blade coating, air knife coating, gravure coating, forward and reverse roll coating, slot and extrusion coating, slide coating and curtain coating. See Cohen, E. in “Modem Coating and Drying Technology,” Chap. 1, VCH Publishers, ISBN 1-56081-097-1.
A brief review of a few patents for coating apparatus and processes is provided below.
Zemlin in U.S. Pat. No. 3,891,785, issued in 1975, discloses a solvent-free, two-part system for forming a polyurethane coating. In Zemlin's process, a rapidly mixed composition is deposited on a moving fabric. The fabric carries the composition under a knife member that limits the amount of composition carried forward and spreads the composition as a layer on the surface. A rolling bank is created in front of the knife member. Shear-thinning is an important property of Zemlin's reaction mixture. As deposited on the moving surface, shear is very low and viscosity of the mixture can reach high values. Shear increases in the rolling bank, and reaches a maximum during the knifing process.
Herzhoff et al. in U.S. Pat. No. 3,893,410 discloses a cascade coater having an inclined flow plane interrupted by a delivery gap for each coating liquid. The liquids flow down to form a coating meniscus and are coated onto an upwardly moving sheet. Similarly, Russell in U.S. Pat. No. 2,761,791 describes a process in which four separate coating compositions are pumped onto slide surfaces down which they flow by gravity to form separate layers that flow down toward a coating bead. The four layers are picked up by an upwardly moving web to form a four layer laminate on the web.
Sitler in U.S. Pat. No. 4,139,662 discloses a coating apparatus in which two compositions are combined in a mixing head and dispensed through a hose. The hose is attached to a conventional reciprocating device that causes the opening of the hose to reciprocate between ends of a trough. The trough is positioned above and perpendicular to a sheet material that travels thereunder. A threaded rod is positioned within the trough and meters the fast-reacting saturant from the trough, through a slot, and onto the moving web of sheet material.
Fast setting chemistry is advantageous for applying polymer coatings; however, a significant problem is polymer setting within the coating apparatus. Mutch in U.S. Pat. No. 3,832,427 discusses the need to design apparatus that can be conveniently opened for manual cleaning, such as brushing and scraping. In Mutch's apparatus, at the end of an operation, the controls are operated to lift the applicator die clear of the table and a solvent and air purge are blown through the mixer and applicator die. Even after this procedure, Mutch remarks that the internal surface of the die passages still need manual cleaning after the die is opened.
Pipken in U.S. Pat. No. 4,345,543 discloses a method and apparatus for coating a moving web. In this method, coating material is fed from a conduit directly into a coating bank. A web passes by one side of the coating bank. Coating material is pulled down between the web surface and the surface of a smoothing film that is backed by a pliable membrane. The force generated by the pliable membrane controls the thickness of the coating.
From the patent and scientific literature, it is clear that there has been a continuing desire for new processes that are more effective, faster, and require fewer resources (i.e., is less expensive). The need for apparatus that is capable of handling fast-setting mixtures and is easy to clean, has long been recognized. Additionally, environmental concerns, such as the desire to reduce or eliminate solvents and avoid spray processes (which can result in airborne pollution), have been of increasing importance over the last 30 years.